Wednesday

Rocket has a book propped on a step to make a ramp and three test spots picked out: wood, rug and grass.

"Same ramp, same ball, same start," he says. "Only the floor changes. Fair test."

Raven lays a tape measure along the wood. "Predict first. Which floor lets it roll farthest?"

"Wood," Rocket says at once. "It is the smoothest."

Nova hovers above the top of the ramp. "What will you do if the first roll wobbles sideways?" she asks.

"Roll it again," Raven says. "Three rolls each. We write all three."

The ball leaves the ramp and whispers across the wood floor. Rocket is already running after it with the tape.

Nova's light flickers. "Measure from the bottom of the ramp, not from the top."

Today you run two tests. First, you roll the same ball from the same ramp onto three different surfaces and measure how far it goes.

Second, you drop a flat sheet of paper and a crumpled sheet from the same height and watch which lands first.

Change only one thing at a time. In the roll test, only the surface changes. In the drop test, only the shape changes.

The ball left the ramp at about the same speed each time. Only the surface changed, so only friction changed.

The surface that stopped the ball soonest pushed back hardest. Friction depends on both materials that touch, the ball and the floor.

If one roll was much shorter than the others, the ball may have wobbled or hit a bump. That is why you rolled three times.

In the drop test, both papers had the same pull of gravity, because the paper was the same. Only the drag changed.

The flat sheet has a large area facing the air, so drag grew large at a low speed. It reached a slow steady speed almost at once.

Careful measuring. Tomorrow you will read a real table of friction numbers for shoes, ice, wood and snow.